Raman scattering is an effect related to the inelastic scattering of photons during their interaction with ions present in a material. Typically, laser radiation of a narrow spectral bandwidth (<1 nm) is used for the stimulation of Raman scattering, and the spectra of the scattered light is measured, whereas the peaks in the spectra are shifted to the red (Stokes shift) or blue (Anti-Stokes shift) side of the spectrum. Under appropriate experimental conditions, according to the positions of the peaks in the spectrum, the materials inside an analyte can be identified.
Surface enhanced Raman scattering (SERS) sensors are usually used to amplify a very weak Raman scattering signal by multitude of times. Such amplification is achieved by the use of plasmonic effects, and especially by the localized surface plasmons. In order to create the right conditions for the interaction between a photon and the surface plasmons, it is necessary to form an array of submicron structures and cover it with a coating of a precious metal, usually gold or silver, as to allow the formation of separate metal islands. The shape, size and position of the structures with respect to each other determine how the plasmons interact with the incident photons of the laser radiation and with those photons, who experienced Raman scattering. The stronger the interaction between plasmons and photons is, the more intense is the amplification of Raman scattering.
SERS sensors can be used in molecular diagnostics and are particularly important for such branches of industry as biotechnologies, drug development, food and soil contamination measurements, forensics, border control, etc.
A plasmon is a quasi-particle described as a quantized oscillation of free electron plasma. A plasmon can couple with a photon forming a new quasiparticle—plasmon polariton. Surface plasmons are surface localized plasmons, which strongly interact with light incident on a surface resulting in a polariton.
In order to create surface plasmons, which interact with the light radiation in the visible spectral range, surface structures, which are the size of tens or hundreds of nanometers, must be formed. The realization of the Surface Enhanced Raman Scattering (SERS) principle requires that the plasmon resonance condition be met.
The most active plasmon resonance phenomena occur on the surface of noble metals, such as gold or silver. Mainly, this is related to the large number of free electrons, present at the metal surface. Due to this reason, in addition to the fact that such metals do not oxidize, they are most often chosen for the production of SERS sensors.
As mentioned previously, the surface structure is particularly important for SERS applications. In case of surface plasmons, the plasmon resonance conditions for small metal elements are strongly dependent on the shape and relative position of those metal particles with respect to each other. Recently, the influence of the gap between two metal particles on the amplification of the electromagnetic field has become especially frequently emphasized. It has been noticed that this parameter (gap size) has the largest impact on the overall amplification of a SERS. The whole field of research on SERS, where the main goal is to detect single analyte molecules, is most often based on this phenomenon. In presence of an especially small gap, localized surface plasmon modes of both metal particles interact, thus forming hybridized modes.
Detectors having a strong Raman scattering amplification are often fabricated by employing nanotechnology principles. Thus, metal islands of the desired size, with small gaps in-between, are obtained. For this reason this research field is often called nanoplasmonics.
In some cases, for the manufacture of SERS devices, methods of laser processing are employed. They do not require physical contact, no additional chemical treatment is necessary after processing, and also during laser processing little additional operations are required. Surface structure patterns, which comprise repeatable features with a period smaller than one micrometer, are formed by using the laser initiated formation of self-organized nanostructures. Few prior-art patents indicate that surface structure patterns, formed by an ultra-short pulse laser, turn-out to be good substrates for SERS sensors possessing high amplification capabilities. Such sensors are often called plasmonic sensors or substrates.
A U.S. Pat. No. 7,586,601, published on Aug. 9, 2009, describes femtosecond laser nanostructured substrates, which are used for the production of SERS sensors. These substrates are made of a semiconductor or a metal. The surface of the material is processed with ultra-short laser pulses, creating ripples or self-ordered nanostructures, and later a noble metal film (e.g., silver or gold) is deposited onto the resulting nanostructured surfaces.
Another U.S. Pat. No. 7,864,312, published on Apr. 1, 2011, describes a substrate for Raman spectroscopy, having a metal coating. The substrate is processed with short laser pulses in order to generate micron-sized or smaller structures on the surface. The structured surface can then be coated with discontinuous metal coating characterized by one or more metallized surface regions and a plurality of surface gaps.
Both prior-art patents describe the formation of smaller than a micrometer size structures by short or ultra-short laser pulses on a surface of a metal or semiconductor. This process is also known as ripple formation. Ripples described in these patents form when ablation occurs on the surface of the material, that is to say that when a material is evaporated directly from the solid state, omitting the melting phase. Nonetheless ripples can be formed only on the surface of specific materials. Most commonly metals or semiconductors are used. It is popular to use silicon, sapphire, germanium, fused silica or similar. When forming ripples on a semiconductor or other crystalline material (sapphire or fused silica) surface, it is necessary to irradiate the material surface with a few thousands of laser pulses. The more pulses are used, the more apparent the ripple structure becomes, yet this process is time consuming. For example, processing a surface area of 1 mm2 with a fairly fast laser (for example when the pulse repetition rate is 600 kHz) can take tens of minutes. Furthermore, further increase in ripple aspect ratio is usually achieved by using aggressive acids, which require very careful handling. This is not convenient and makes manufacturing costly.
Also, the material, used in earlier solutions, is expensive. This results in a prime cost of a sensor and complicates production operations.